1. Field of the Invention
The present invention relates to magnetic resonance (MR) imaging, and more particularly relates to a method and apparatus useful for developing high resolution MR images of an object in motion.
2. Description of the Related Art
It is well known that Magnetic Resonance (MR) images can be degraded by motion during the imaging scan. One particular exemplary application of MR imaging where motion is an issue, is in the field of orthopedics, where physicians find value in viewing not only static images of joints, but also images of the joint while in motion (commonly referred to as “kinematic studies”). In order to provide such images, several devices exist for adjusting and then fixing the position of a joint so as to image it at selected ones of a plurality of different fixed positions. Use of such mechanical devices is undesirable because of the time it takes to repeatedly adjust and then fix the position of the joint to successive new positions, and to individually acquire the MR images at each of those fixed positions. Although movies of joint motion can be created with such devices, such kinematic studies do not demonstrate the dynamic effects of normal movement, such as muscle contraction against resistance and joint compression.
One known magnetic resonance (MR) method for acquiring images of a moving joint uses a very fast pulse sequence of ordinary and well known design to acquire single images in less than one tenth of a second, yielding so-called “real time” imaging of the joint motion. However, the speed of the acquisition reduces the resolution and signal-to-noise ratio (SNR) of the images, and joint motion during this fraction of a second can still cause a degradation of image quality.
Another known method of MR imaging acquires data only at several specific rotational positions as the joint is repeatedly flexed by a motorized apparatus. During each flexion, only a portion of the required data is acquired at each joint position, this acquisition being performed with synchronization of the MR imaging pulse sequence with the motorized drive. The data from several flexions are combined to form complete images at the specified rotational positions. Because a portion of the data may be acquired in a shorter time window than a full data set, blurring is reduced (See for example Shah's U.S. Pat. No. 5,154,178). However, it is not convenient to drive the motion of the joint in this synchronous manner, and the images do not show the natural positions of the tissues as the joint works against resistance.
Another known method of MR imaging triggers a full or partial acquisition of MR data as the joint is flexed, this triggering being initiated by a position measuring device. Because motion during this acquisition period will blur the resulting images, the period is kept short, and the MR imager is halted shortly after the onset of the triggered acquisition. The imager pauses until the position measuring device signals the next desired position (See for example De Boer's U.S. Pat. No. 5,931,781). However, the natural irregular motion of the joint makes these pauses irregular in duration, allowing the tissue magnetization to relax back to its equilibrium value for an irregular time. This changes the amplitude of the acquired signal in an irregular manner, creating image artifacts.
Another known method of MR imaging acquires repeated copies of one data subset during one cycle of tissue motion, this acquisition utilizing one set of position-encoding magnetic field gradients and a steady rhythm which is asynchronous with the cycle of tissue motion. This rhythmic acquisition continues into the next cycle of motion, where, with a different set of position-encoding magnetic field gradients, repeated copies of a second data subset are acquired. This technique is commonly used to acquire one or several position-encoded “Fourier lines” of the beating heart per cycle, the start of each cycle announced by the “R-wave” of the electrocardiogram. Although the varying durations (“periods”) of each cycle are known, the exact motion of the, for example, heart tissue during these cycles must be assumed. One such assumption would be that the complex motion of the heart tissue stretches or contracts proportionally with the period of the cycle. After making this or some other assumption to correlate time with position, the data are retrospectively sorted into groups that are presumed to represent specific spatial positions of the heart tissue. The accumulated data in each group are then reconstructed to form images. The steady rhythm of the data acquisition maintains the tissue magnetization in a steady state to reduce image artifacts (See for example Pelc et al's U.S. Pat. No. 4,710,717). However, this technique is not directly applicable to joint imaging, because it assumes that the tissue moves in a predictable pattern from one cycle to the next, even if the period of the cycle changes. During joint imaging, the patient may move the joint in an unpredictable manner following initiation of movement.
Because the technique of Pelc acquires data asynchronous to the movement of the heart, it is clear that, even after a moment in time is estimated to represent a particular spatial position of the heart, it is not possible retrospectively to select an MR data subset that was acquired at precisely this moment. To reduce image artifacts created by this temporal mismatch, two data subsets are linearly interpolated: one is acquired just before this moment, and the other is acquired just after this moment. Only data acquired with the same position-encoding gradient pulse or pulses are interpolated.